Switching converters are widely used due to their high efficiency and simple internal structure. Many control modes could be used to control switching converters, such as constant on time (COT) control mode, peak current control mode, and average current control mode. Among these control modes, COT control mode is getting more and more popular per its fast transient response, simple structure and smooth switch of operation mode. But, for a switching converter with COT control mode, the switching frequency is easily affected by the change of load, input voltage or output voltage. Variation of the switching frequency will cause EMI (electromagnetic interference), EMC (electromagnetic compatibility), noise and other problems. Designing control circuit for switching converter, especially for switching converter with COT control mode, becomes a big challenge.